1. Field of the Invention
This invention relates to an output control circuit and output control method for monitoring a plurality of internal signals existing in an integrated circuit such as a large-scale integrated (LSI) circuit.
2. Description of the Related Art
In recent years, LSI circuits having high levels of integration have been realized due to remarkable advances in semiconductor technology. A central processing unit (CPU), memory elements, several hundreds of thousands of logic circuits and the like, which previously had been configured as separate circuits, can now be integrated into a single chip.
However, this higher level of integration makes it extremely difficult to evaluate or test LSI circuits. Namely, if a system that incorporates large-scale integration does not operate properly, in contrast to the case where the CPU, memory elements, logic circuits and such are configured on separate chips, thereby enabling easy observation of the signal lines between individual semiconductor devices and relatively simple discovery of the location of a defect, in the case of single chip solutions, internal signal lines are not easily observable.
To address this issue, Japanese Laid-Open Patent Publication (Kokai) Nos. S64-41257 and H5-302961 proposed circuits for monitoring internal signals in which one signal selected from a plurality of internal signals is output from a designated output terminal.
However, in the circuits disclosed in the above publications, selected internal signals are fixed to respective designated output terminals in one-to-one correspondence. Thus, even if a plurality of output terminals are provided, the number of simultaneously observable internal signals is limited by the number of output terminals provided and the selection circuit that selects the internal signals. If a large number of internal signals are to be observed simultaneously, it is necessary to provide the same number of output terminals as the internal signals to be observed.
It is an object of the present invention to provide an output control circuit and output control method that enable any arbitrary internal signal to be specified and output from any output terminal, and hence enable the monitoring of a large number of internal signals even if only a small number of output terminals are provided.
To attain the above object, a first aspect of the present invention provides an output control circuit provided in an integrated circuit, comprising a plurality of external output terminals that output a plurality of signals existing in the integrated circuit, and a plurality of signal output circuits to which the plurality of internal signals are mutually input, wherein each of the signal output circuits comprises a signal group select circuit that selects a predetermined signal group from the plurality of internal signals, a logic circuit that carries out logical operations on the selected signal group, and an external output circuit that outputs signals resulting from the logical operations via one of the external output terminals.
In a preferred form of the first aspect, the logic circuit comprises a plurality of individual logic circuits that individually carries out logical operations on signals forming the selected signal group, and a logic select circuit that selects one individual logic circuit from the plurality of individual logic circuits, and the external output circuit outputs a signal resulting from a logical operation carried out by the selected individual logic circuit via one of the external output terminals.
The logic circuit may preferably a type that carries out a logical OR operation that causes a predetermined output to be output when at least one of signals forming the selected signal group is in an active state, or a type that carries out a logical AND operation that causes a predetermined output to be output when all signals forming the selected signal group are in active states.
In a preferred embodiment of the first aspect, the logic circuit comprises at least one logical OR circuit that carries out a logical OR operation that causes a predetermined output to be output when at least one of signals forming the selected signal group is in an active state, at least one logical AND circuit that causes a predetermined output to be output when all signals forming the selected signal group are in active states, and an operation select circuit that selects either the logical OR circuit or the logical AND circuit.
To attain the above object, a second aspect of the present invention provides an output control method of outputting a plurality of internal signals existing within an integrated circuit via external output terminals, comprising a selection step of selecting a predetermined signal group from the plurality of internal signals, using a plurality of signal output circuits to which the plurality of internal signals are mutually input, a logical operation step of carrying out logical operations on the selected signal group, and an output step of outputting signals resulting from logical operations via the external output terminals.
In a preferred form of the second aspect, in the logical operation step one logic circuit is selected from a plurality of individual logic circuits that carries out individual logical operations on signals forming the selected signal group, and in the output step a signal resulting from a logical operation carried out by the selected individual logic circuit is output via one of the external output terminals.
Preferably, in the logical operation step a logical OR operation is carried out to cause a predetermined output to be output when at least one of signals forming the selected signal group is in an active state. Alternatively, in the logical operation step a logical AND operation is carried out to cause a predetermined output to be output when all signals forming the selected signal group are in active states.
In a preferred embodiment of the second aspect, the logical operation step comprises a logical OR step of carrying out a logical OR operation that causes a predetermined output to be output when at least one signal forming the selected signal group is in an active state, a logical AND step of carrying out a logical AND operation that causes a predetermined output to be output when all signals forming the selected signal group are in active states, and an operation select step of selecting either the logical OR step or the logical AND step.
According to the present invention constructed as above, any arbitrary internal signal can be designated for outputting from any external output terminal. Therefore, by providing the same number of signal output circuits as the number of internal signals that need to be monitored simultaneously, a large number of internal signals can be monitored even if there are only a small number of external output terminals.
The above and other objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.